Project Summary Prostate cancer (PCa) incidence and mortality remain significantly higher in African American (AA) men compared to Caucasians (CA). Although socioeconomic factors may contribute to such differences, recent studies suggest that biological factors could potentially explain racial disparities (RD) in PCa. A few high- penetrance genes (HPG) have been found to predispose to high risk of PCa in AA men. These include EphB2 mutations. EphB2 is a tyrosine kinase (RTK) receptor that binds to the Ephrin ligand and initiates bidirectional signals that affect both receptor (forward signaling) and Ephrin-expressing cells (reverse signaling). Our preliminary observations suggest a potential supportive role of Ephrin signaling activation within stromal cells in the tumor microenvironment (TME) due to the pathogenicity of these EphB2 genetic alterations. However the functional consequences of EphB2 alterations on prostate cancer cell biology have not been experimentally validated. To better understand the intrinsic mechanism(s) responsible for these pathogenic effects we hypothesize that EphB2 alterations commonly seen in AA men may induce aberrant ?forward signaling? reducing its tumor suppressor function in cancer cells and abnormal ?reverse signaling? increasing the activation of stromal in the TME, overall resulting in support for PCa progression. This proposal seeks to understand the downstream molecular effectors of disrupted Eph-Ephrin signaling in AA men in both tissue compartments (epithelium and stroma) on PCa progression. The long-term goal is to identify novel targetable molecules within the cancer and/or stromal compartment that can be used to improve diagnosis and treatment of patients at high-risk of developing aggressive PCa disease in AA men. Three complementary areas will be investigated: 1. Determine the functional effects of racial EphB2 alterations on PCa progression. We will study the functional consequence of common EphB2 alterations seen in AA men with PCa, on proliferation and motility of PCa cells in vitro and in vivo. The role of EphB2 in PCa cell biology is unknown. 2. Determine the role of Ephrin reverse signaling on CAF activation in the TME of AA men. We observed increased expression of EphB2 ligands in the TME of AA compared to CA men suggesting a dysfunctional regulation of Ephrin-reverse signaling. In addition we observed that prostate stromal cell from AA patients with PCa induce increased proliferation and motility of PCa cell lines in vitro and in vivo. 3. Evaluate the utility of targeting the Eph-Ephrin signaling to inhibit PCa progression. It has been shown in experimental models that normalizing Eph-Ephrin signaling can reduce colon cancer progression. The utility in PCa has not been previously studied. Using an in vivo PCa model, we will assess the feasibility of targeting Eph-Ephrin signaling frequently altered in AA men to inhibit PCa tumorigenesis.